The error-free function of modules in a bus system with a central unit is tested. First, all (error-free) modules are set into a silent operating mode in response to an agreed silence-command, and then in this silent operating mode it is tested whether all modules still remain silent, i.e. do not transmit data, in response to an agreed data request allocated to these modules. A module that nonetheless transmits data is recognized as faulty and is deactivated. Subsequently, all modules except for a module to be tested are set into the silent operating mode, and then it is tested whether this test module transmits its data in response to the agreed data requests allocated to it and remains silent in response to the data requests not allocated to it. Additionally, the position of the return-transmitted data can be determined and compared with a nominal position.